This invention relates to a fluorescent borescope for providing a full color video image of a generally inaccessible target that fluoresces when illuminated with ultraviolet light, and more particularly to an apparatus for measuring the size of fluorescent targets viewed on the image display of the borescope.
Borescopes have been provided in the prior art for realizing a black-and-white or full color video picture of a generally inaccessible target that is situated within a cavity and therefore cannot be viewed using direct vision. They are commonly used to identify cracks or defects in the structure of remote, inaccessible surfaces of industrial machinery such as pipes within steam generating equipment, aircraft, and other equipment where structural integrity is important to safe operation.
Fluorescence penetrant borescopy is a well known technique for finding cracks or defects. To perform this technique, a chemical compound that acts as a penetrant is applied to the surface to be viewed and time allowed for penetration into surface discontinuities. After removal of excess penetrant the surface is treated with a developer that has a blotting action, so that the compound will be drawn out from the discontinuities onto the surface where it fluoresces when exposed to ultraviolet light. The fluorescence can then be visualized as an indication through a rigid or a flexible borescope.
Heretofore when using fluorescence penetrant borescopy, all attempts to measure the image on the video display to determine the size of the target being viewed have utilized mechanical accessories. One approach requires the placing of a known scale adjacent to the image to be measured for a comparison measurement. Alternatively a physical standoff can be provided over the lens on the end of the borescope insertion tube at which point the magnification is known. By adjusting the end of the borescope until it just touches the target to be viewed at the standoff, the image will appear on the screen at a known magnification, whereupon it can be measured and its precise size determined. While these methods of measurement are effective, they remain somewhat awkward and painstaking, and improvements have been desired.
It is taught in U.S. Pat. No. 4,980,763 to Lia of common assignee herewith, that targets illuminated through a borescope by a general illumination source can be measured by creating an auxiliary image having a known characteristic, such as a shadow, and projecting it on the target in such a way that its position or size changes in proportion to the distance of the image sensing head from the target. An auxiliary illumination source means is commonly used to generate the auxiliary image. A precalibrated target magnification and distance overlay for the geometry of the borescope is placed on the video display to indicate the magnification factor for the physical measurement of the target on the display screen. Alternatively electronic calibration and measurement circuitry can be provided. This is known as the shadow measurement technique. The art of shadow measurement has been more recently advanced by modifications of the illumination system in borescopes. A typical improved device is found in a co-pending application, Ser. No. 573,870, entitled "Apparatus and Method for Selecting Fiber Optic Bundles in a Borescope", and assigned to the Assignee of the present Application.
In the prior art devices the target to be viewed is illuminated by light transmitted through at least two separate fiber optic illumination bundles, and means are provided for selectively illuminating one or the other, or both of the bundles, depending on the illumination desired on the target to be viewed. A general light source operates through a first fiber optic bundle and illuminates the target. An auxiliary fiber optic bundle is typically configured as a thin strip extending across the first fiber optic bundle, and an opaque index element or bar is positioned a preassigned spaced distance in front of the termination of the auxiliary fiber optic bundle. When the auxiliary fiber optic bundle is actively carrying light, a shadow is cast by the opaque index element onto the target being viewed.
Shadow measurement has not been practicable with fluorescent borescopes because of insufficient illumination. To achieve acceptable accuracy, relatively small optical fiber bundles that emit correspondingly small amounts of light have been used in the prior art to generate the shadow image. Furthermore, light losses occur in the filters required for certain fluorescent optical instruments. These factors and small target indication sizes combine to limit the visibility of the shadow that is projected.